Fig. 16.12 ) also propagate the slow response, whose conduction characteristics differ quantitatively from those of the fast response. The threshold potential is approximately −40 mV for the slow response, and conduction is much slower than for the fast response. The conduction velocities of the slow response in the SA and AV nodes are approximately 0.02 to 0.10 m/sec. The fast-response conduction velocities are approximately 0.30 to 1.00 m/sec for myocardial cells and 1.00 to 4.00 m/sec for the specialized conducting (Purkinje) fibers in the ventricles. Slow responses are more readily blocked than are fast responses; that is, conduction ceases before the impulse reaches the end of the myocardial fiber. Also, fast-response fibers can respond at repetition rates that are much faster than those of slow-response fibers.
Fig.16.13 alsotakeplaceinthecardiactissueofpatientswithcoronaryarterydisease.Whenregionalmyocardialbloodflowisdiminished,thesupplyofO2andmetabolicsubstratesdeliveredtotheischemictissuesisinsufficient.TheNa+,K+-ATPaseinthemembraneofcardiacmyocytesrequiresconsiderablemetabolicenergytomaintainthenormaltransmembranegradientsofNa+ andK+ .Whenbloodflowisinadequate,theactivityofNa+,K+-ATPaseisimpaired,andtheischemicmyocytesgainexcessNa+ andloseK+ tothesurroundinginterstitialspace.Asaconsequence,[K+]intheextracellularfluidsurroundingtheischemicmyocytesiselevated.Suchchangesinextracellular[K+]may,ifsufficientlylarge,disturbcardiacrhythmandconductioncritically(see
Fig.16.13
Because of the rapid development of artificial pacemakers and other electrical devices for correcting cardiac rhythm disturbances, detailed knowledge of cardiac excitability is essential. The excitability characteristics of various types of cardiac cells differ considerably, depending on whether the action potentials are fast or slow responses.
Once the fast response has been initiated, the depolarized cell is no longer excitable until it has partially repolarized (see
Fig. 16.1A ). In the fast response, the interval from the beginning of the action potential until the fiber is able to conduct another action potential is called the effective refractory period which extends from the beginning of phase 0 to a point in phase 3 at which repolarization has reached approximately −50 mV. At approximately this value of Vm, many of the fast sodium channels have transitioned from the inactivated state to the closed state. However, the cardiac fiber is not fully excitable until it has been completely repolarized. Before complete repolarization (i.e., during the relative refractory period), an action potential may be evoked only when the stimulus is stronger than a stimulus that could elicit a response during phase 4.
When a fast response is evoked during the relative refractory period of a previous excitation, its characteristics vary with the membrane potential that exists at the time of stimulation (
Fig. 16.14).
The later in the relative refractory period that the fiber is stimulated, the greater are the increases in the amplitude of the response and the slope of the upstroke because the number of fast sodium channels that have recovered from inactivation increases as repolarization proceeds. As a consequence, propagation velocity also increases the later in the relative refractory period that the fiber is stimulated. Once the fiber is fully repolarized, the response is constant no matter what time in phase 4 the stimulus is applied.